Sideway and sideway

These past three days I have been thinking about flying in the wind, sideslip...
All of these thoughts come when I was at Barnes & Nobles in Mall of America last Sunday and read the Aircraft Flying Handbook from FAA.
I read one sentence in the book saying (may not be an exact quotation):

As an aircraft flies in the wind, it is moving together with the wind. Therefore, when an aircraft flies in the wind, it's prevalent that the pilot makes a continuous wind correction to maintain course.

It's kind of difficult for me to accept that air is just like water. Now, when a boat moves in a water stream, it will follow the stream's flow. It will have exactly the same velocity as the water flow. It's very easy to see that. The boat when not rowed will always maintain zero relative velocity with respect to the flow.

But how do I see that in airplane? I could not imagine an aircraft floating..

Well, theoretically when the headwind is strong enough, an aircraft can lift off the ground without any thrust produced by the engine and no relative movement on the ground. This is very easy to see because then there is an airspeed seen by the wing. Meaning that with respect to the air, the aircraft is moving forward. But it's not moving with respect to ground...

Can you see the difference now? In the boat case, it is not moving with respect to the water, but in the aircraft case, it is not moving with respect to the ground!

How how how....

Well, I'm gonna rush to the class now. I will post something again later. :)


..continuing...
Ok..well, I think I need to rephrase my question.
If it's true that an aircraft drift to the right 5 miles an hour when a 5 knots wind blow from the left, will a Cessna 172 moves backward 120 miles an hour (with respect to the ground) an hour when there is no thrust produced when encountering a 120 knots headwind?

Well, I guess the bottom line is still, air is eventually a fluid just like water. So if a boat follows the river's flow, so does an aircraft with the wind.

I do think that I get all of this thing mixed up. LOL!

So, let me sort things out now...

Well, surely ANYTHING that makes no resistance (for example - thrust produced by the engine in the case of an aircraft) or whatsoever when it is blown by a headwind of 120 knots will surely go backward at 120 miles an hour. Starting from rest, it will speed up to the 120 knot backward because of the headwind. Eventually, there is no relative velocity between the object with the air. So in the case of a power-off Cessna 172 blown by a headwind of 120 knots: well, it will certainly have lift but eventually when no other mode of resistance (e.g. thrust) is made by the aircraft, it will fall down (although it will then gain speed again as it loses altitude and come back flying. Let that discussion alone) - just like a rock. And if we are to look closer of the mechanism, it's the drag created by the relative motion between the body and the air that makes the body "flows" together with the air.

So, still, in order to fly/lift-off, the aircraft still needs to produce power that overcome that drag although it may result in the aircraft not moving with respect to the ground (in the case of Cessna 172 that cruises at 120 knots).

In the crosswind case, I think I should recall again that the wind acts simply just like water. So, if there is a crosswind during cruise, that will surely drift your aircraft sideways, simply because it is moving together with the air. Consider a pure crosswind situation, the forward velocity is simply your velocity with respect to the air. So, for the ground speed/track, they simply add up "vectorialy" or so to say. Up to this point, we are neglecting anything about the side force or any weathercock stability that may turn the aircraft into the wind. But yes, if we are to look closer again, it is the drag (or in this case side force) that has moved the body together with the wind.

Supposed you are flying a Cessna at 50 knots (forward) airspeed in a 20 knots headwind.
Let's say that the forward motion is x-axis and the crosswind direction is y-axis.

We could say that the aircraft is moving at the airspeed of 50 knots forward (x-direction) and no airspeed in the y-direction.
And the groundspeed is then, 50 knots forward (x-direction) and 20 knots airspeed in the y-direction, or about 22 degree sideways (if I am to take the arctangent) - again this is groundspeed!

So, in order to maintain a straight ground track, I think there are two possible ways to overcome the sideways motion:
1. By orientating your aircraft's heading in such a way that the vector component of your velocity that is parallel to (cross)wind direction cancels the wind's speed. In that case, you are flying straight on the ground. I think, this is what is called as the wind correction angle method.

Or,

2. By means of rudder application. Rudder can be used to zero the side force. Sufficient left rudder could produce enough side force to maintain the aircraft not drifting to the left when a wind from the right blows, couldn't it? But then, due to the cross coupling of the aerodynamic moment, you may need sufficient aileron deflection as well.

This method is totally an aerodynamic waste because it creates a lot lot of drag.

Another way is to equip the aircraft with a heaven-knows-what equipment that can give an airspeed of 20 knots in the negative y direction (that is opposite to the crosswind).
The groundspeed will then become 50 knots forward (x-direction) and 0(zero) knots in the y-direction - or simply said, it is on track!

Well, again, this is a heaven-knows-what equipment, so this is purely a theoretical discussion and perhaps basic vector mechanics. Imagine you can have a propeller mounted sideway. I think the side prop can always "push" the aircraft so that it flies in the desired direction (ground track) without any heading correction. With today's knowledge and technology

But I think, with the advance of thrust vectoring method, there is a possibility that one day, we would have wind correction in this manner. Perhaps I should do my thesis on this one? LOL!

My discussion reference with some pilots can be found in the IVAO-HQ Training forum or simply click here.